1. Field of the Invention
The present invention generally relates to an image forming apparatus group having a plurality of image forming apparatuses each employing an electrophotographic process and having different process speeds.
Here, an image forming apparatus includes an electrophotographic copying machine, an electrophotographic printer, such as an LED printer or a laser beam printer, and electrophotographic facsimile terminal equipment, each employing an electrophotographic process, and an image forming apparatus or the like using an electrostatic recording process.
2. Related Background Art
In an image forming apparatus using an electrophotographic process, such as a copying machine or a laser beam printer, light corresponding to image information is applied to a surface of a photosensitive member serving as a charged image bearing member to form a latent image on the surface of the photosensitive member by exposing means. A developer (toners) as a recording material is supplied to the latent image by a developing apparatus to visualize the latent image to obtain a developed image (toner image), and the developed image is transferred from the surface of a photosensitive member to a recording sheet as a recording medium, thereby forming an image on the recording sheet.
With respect to a developing apparatus using dry one-component development, various kinds of developing apparatuses have been proposed. Then, the following method is used as the most general method. That is, this method is employed such that a uniform toner layer is formed on a developing roller as a developer carrying member by a regulating member, and a developing bias voltage having an A.C. component and a D.C. component is applied to the developing roller to carry out the developing based on a difference in electric potential between an electrostatic latent image on a surface of a photosensitive member and the developing roller.
In the above-mentioned developing apparatus, a toner container as a toner containing portion is linked to a developing portion including the developing roller and the regulating member, and the toners are consumed as images are formed. The developing apparatus having the toner container, the photosensitive member, a charging means for charging the surface of the photosensitive member with electricity in order to form the latent image on the surface of the photosensitive member, and the like are constructed integrally with one another in the form of a process cartridge in many cases. Thus, when the toners are completely consumed, a user exchanges the old process cartridge for a new one, thereby allowing an image to be formed again. To know at any time how much toner, which is able to be supplied for image formation, remains within the process cartridge, a developer amount detecting system capable of successively detecting a toner remaining amount level is provided in the process cartridge or an image forming apparatus main body in some cases. Of such cases, in a case of a method of detecting a toner remaining amount by measuring an electrostatic capacity between electrode members, an added circuit is relatively simple in configuration, and high precision measurement is obtained. Thus, several methods are proposed. U.S. Pat. No. 5,987,269 discloses a method of successively detecting a toner amount by measuring an electrostatic capacity between sheet metal-like electrodes. Also, U.S. Pat. No. 6,415,112 proposes a method of successively detecting a toner amount by measuring an electrostatic capacity between a sheet metal-like electrode member provided within an image forming apparatus or within a process cartridge, and a developing roller.
In recent years, high speed promotion of a process speed and long life promotion of a process cartridge have been required for an image forming apparatus in which the process cartridge is adapted to be mounted to an image forming apparatus main body as described above. Thus, a toner amount (toner capacity) filled in a toner container has tended to increase. Here, the process speed generally means a rotational speed of a photosensitive drum. Thus, the number of recording sheets on which images are formed per hour further increases as the process speed becomes higher. Under such circumstances, when excellent images are intended to be maintained for a long term from the beginning of use of the process cartridge to a time point when the toner is completely consumed, various problems arise
A long term change of the developing characteristics due to a change in a state of the toners is important. In particular, in order to stably maintain an image density level, heretofore, various proposals have been made.
Japanese Patent Application Laid-Open No. S59-184375 proposes a method in which, in order to supplement a reduction in the image density level when toner is supplied, an effective value of a developing bias voltage is controlled in correspondence to a toner remaining amount within a toner container. In addition, Japanese Patent Application Laid-Open No. S56-62275 proposes a method in which a peak-to-peak voltage of a developing bias voltage is changed in correspondence to the number of sheets of copies to supplement an image density level and to simultaneously reduce fog. In addition, Japanese Patent Application Laid-Open No. 2002-244365 discloses a control method in which a peak-to-peak voltage and a frequency of a developing bias voltage are changed in correspondence to an amount of remaining toner and the environment, thereby supplementing an environmental fluctuation and a long term change of developing characteristics. Also, Japanese Patent Application Laid-Open No. S57-19769 proposes a method in which a developing bias voltage is controlled in correspondence to the dead time of a developing apparatus and the number of sheets of copies, thereby supplementing the rise of developing characteristics after stoppage for a long time. Moreover, Japanese Patent Application Laid-Open No. H11-272048 proposes a method in which a developing bias voltage is controlled in correspondence to the dead time of a developing apparatus and the environment, thereby supplementing the rise of developing characteristics after stoppage for a long time.
In addition, U.S. Pat. No. 6,134,396 proposes a method including changing a peak-to-peak voltage of a developing bias voltage based on use history information of a developer. Also, U.S. Pat. No. 6,597,876 discloses a method including changing process conditions based on information inherent in a process cartridge which is stored in a storage medium mounted to a process cartridge.
However, recently, an image forming apparatus group has been required which can cope with various applications of users by allowing a process cartridge common to a plurality of image forming apparatus main bodies different in their process speeds to be used.
When the common process cartridge can be used in the image forming apparatuses different in their process speeds, it is important that even when the process cartridge used in a certain image forming apparatus is used in another image forming apparatus in the middle of being used, an excellent image is obtained without any fluctuation in image level.
However, when the same process cartridge is used in an image forming apparatus main body (f) having a process speed of Vf mm/sec and an image forming apparatus main body (s) having a process speed of Vs mm/sec lower than Vf, there is encountered a problem that as shown in FIG. 3, a difference in image density becomes large along with an increase in the amount of used toner (a decrease in the amount of remaining toner).
It is considered that this problem is caused by the influence of a long term change in particle size distribution of toners, i.e., a so-called selection phenomenon due to the fact that the toners, each of which has a relatively small particle diameter and which has an excellent developing property, are preferentially consumed. At the beginning of use, both the toners each having a small particle diameter and the toners each having a large particle diameter are contained in the developing apparatus within the process cartridge. Since the toners each having a small particle diameter are sensitive to the developing bias voltage and have an excellent developing property, the development can be carried out at nearly the same density for a latent image irrespective of a high process speed and a low process speed when a latent image is developed with those toners. However, when the toners are continuously used for a long period of time, only the toners each having a large particle diameter remain since the toners each having a small particle diameter are preferentially consumed. The toners each having a large particle diameter have an inferior developing property to the toners each having a small particle diameter, and hence have a high process speed. Thus, in the image forming apparatus in which a short period of time is required for a photosensitive drum to pass through a developing area, that is, an area facing the photosensitive drum and the developing roller, the development is not sufficiently carried out, and hence sufficient density is hard to obtain. Consequently, when the process cartridge in the middle of being used in an image forming apparatus is used in another image forming apparatus different in process speed from the image forming apparatus, the difference in image density between those two image forming apparatuses becomes large.
It is an object of the present invention to reduce density differences among images output from a plurality of image forming apparatuses even when a cartridge which is commonly and detachably mountable to a plurality of image forming apparatus main bodies is changed and used among a plurality of image forming apparatus main bodies in an image forming apparatus group including a plurality of image forming apparatuses that are different in process speed from one another.